[builders] Valid use step directives are missed after raw backticks #3900

Bug

@workflow/builders 4.1.10 can report hasUseStep: false for a valid use step directive when a raw triple-backtick sequence appears earlier in the file and a template literal appears after the directive.

Minimal reproduction

// Markdown fence marker: ```
export async function brokenStep() {
  "use step";
  return `ok`;
}

The same result occurs when the first line is a regex literal:

const openFence = /```/;

export async function brokenStep() {
  "use step";
  return `ok`;
}

Run the detector:

import { readFileSync } from "node:fs";
import { detectWorkflowPatterns } from "@workflow/builders";

const source = readFileSync("step.ts", "utf8");
console.log(detectWorkflowPatterns(source));

Actual

{
  hasUseWorkflow: false,
  hasUseStep: false,
  hasDirective: false,
  hasSerde: false
}

Expected

hasUseStep should be true. Comments and regex literals elsewhere in the file should not affect directive discovery.

Notes

The current detector masks template-literal-looking spans before masking comments. The template-literal regex runs against raw source, so it can pair backticks from comments or regex literals with a later template literal and mask the real directive between them.

Please add regression cases for both examples above.